FIG. 1A shows an underside plan view of a conventional quad flat no-lead (QFN) package utilized to house a semiconductor device. FIG. 1B shows a cross-sectional view taken along line B-B′, of the conventional QFN package of FIG. 1A, positioned on a PC board.
QFN package 100 comprises semiconductor die 102 having electrically active structures fabricated thereon. Die 102 is affixed to underlying diepad 104a portion of lead frame 104 by die attach adhesive 106. The relative thickness of the die and lead frame shown in FIG. 1B, and all other drawings of this patent application, is not to scale. Lead frame 104 also comprises non-integral pin portions 104b in electrical communication with die 102 through bond wires 108. Bond wires 108 also allow electrical communication between die 102 and diepad 104a. 
Plastic molding 109 encapsulates all but the exposed portions 104a′ and 104b′ of the lead frame portions 104a and 104b, respectively. For the purposes of this patent application, the term “encapsulation” refers to partial or total enveloping of an element in a surrounding material, typically the metal of the lead frame within a surrounding dielectric material such as plastic.
Portions of the upper surface of lead frame 104 bear silver (Ag) 105 formed by electroplating. The lower surface of lead frame 104 bears a layer of Solder; Lead-Tin, Tin or, Tin Alloys for Lead-free products. Both the lower and upper surfaces of lead frame 104 bears a layer of Ni—Pd—Au or Ni—Au 107 formed by electroplating for the pre-plated lead frames for QFN.
QFN package 100 is secured to traces 110 of underlying PC board 112 by solder 114 that preferably has the rounded shape indicated. The electrically conducting properties of solder 114 allows electrical signals to pass between lead frame portions 104a and 104b and the underlying traces 110.
The QFN packages just described are typically Fabricated as part of a larger, continuous metal matrix defining the diepads and leads. Individual packages are then singulated from the matrix by physical means such as sawing to sever the metal connections.
FIG. 1C shows a plane view of a matrix 196 of QFN packages prior to singulation. FIG. 1D shows an enlarged cross-sectional view showing the diepad and leads of an individual package taken along the line 1D-1D′ of FIG. 1C. FIG. 1D shows that the molded matrix 196 of packages is supported by saw tape 199.
FIG. 1E shows a cross-sectional view of the inter-package region of FIG. 1C, taken along the line 1E-1E′. FIG. 1E shows that leads 104b of adjacent packages are formed from a common, integral piece of metal known as a tie-bar 198. Conventionally, the individual packages are singulated from the common matrix by physical sawing along “saw streets” 192 in inter-package regions comprising these common metal tie-bars. The line 1E-1E′ represents such a saw street.
FIG. 1F shows a simplified cross-sectional view along line 1E-1E′ of the package matrix of FIG. 1C with pre-plated Ni—Pd—Au lead frames, after an initial step of the conventional sawing package singulation process. Initial saw cut 160 is of sufficient width 162 to remove the entirety of the Ni/Pd/Au plating 105 and a portion of the underlying Cu alloy of the tie-bar 198 along the saw street.
FIG. 1G shows a simplified cross-sectional view along line 1E-1E′ of the package matrix of FIG. 1C, after continued sawing during the package singulation process. FIG. 1G shows complete removal of the Cu alloy of the connecting metal tie-bar, with the adjacent packages held together in the matrix only by common plastic molding 109 supported by underlying saw tape 199. Because the leads 104b of individual packages of FIG. 1G no longer share a common piece of metal, they are electrically isolated from one another.
FIG. 1H shows a simplified cross-sectional view along line 1E-1E′ of the package matrix of FIG. 1C, after a final step of the conventional sawing package singulation process. In FIG. 1H, sawing is continuous through the plastic mold 109, thereby completely physically separating the individual packages, which now remain bound together only by underlying saw tape 199. Singulated packages 100 may now be plucked from saw tape 199 for mounting within an electronic apparatus.
The conventional package singulation process illustrated above in FIGS. 1C-1H allows for the fabrication of packages by mass-production. However, this conventional singulation process offers a number of potential disadvantages.
One such disadvantage is relatively low throughput. Specifically, the process of sawing through metal material requires considerable care, to ensure continuing electrical integrity of semiconducting structures housed within the packages. In particular, rapid sawing of the metal can generate residual electrical charge that can short-out or otherwise disrupt the electrical connections carefully fabricated within the packages. Accordingly, steps of the conventional package singulation process that involve the sawing of metal are performed slowly and under carefully controlled conditions, reducing throughput of the overall package singulation process.
Therefore, there is a need in the art for improved techniques for fabricating semiconductor device packages.